Colorectal cancer occasionally metastasizes to the anal canal. Studies on the matter are dated and it is timely to review the current evidence. We report a case of a 52-year-old male with rectosigmoid adenocarcinoma and a metastatic anal nodule at the scar of a previously treated perianal abscess. Comparison of histological features and mutational profiling on next-generation sequencing (NGS) determined the anal lesion as metastatic adenocarcinoma arising from the rectosigmoid primary. Differentiating anal implantation metastases from a synchronous primary anal adenocarcinoma has therapeutic implications. CK7, CK20 and CDX2 immunohistochemistry can be helpful, although these have limitations in differentiating certain cell lines. In this case, NGS supplemented standard immunohistochemistry and facilitated the diagnosis of anal metastases. This case highlights unique considerations in diagnostic evaluation and management of anal implantation metastases, and the utility of NGS in determining the pattern of spread in metastatic colorectal cancer.
Keywords: colorectal cancer; implantation metastases; molecular diagnostics; perianal disease.
© The Author(s) 2025. Published by Oxford University Press and JSCR Publishing Ltd.